1. Field of the Invention
The invention relates to a process for the butt-welding by means of a laser beam of especially deep-drawable steel sheets or steel strips galvanized at least on one side and having essentially rectangular cut edges, in which the welding seam is protected against corrosion.
Fabricators of galvanized sheets require for specific purposes, for example for molding plates to be produced by deep-drawing, sheets which have a width for which currently available galvanizing plants are not designed. They have therefore resorted to butt-welding two or more sheets together. The production of such sheets by conventional butt-welding methods makes it necessary to rework the welding seam, because it is impossible to obtain a welding seam flush with the sheet and suitable for further processing (deep-drawing), and because there is no corrosion protection in the region of the welding seam. In particular, in the conventional processes, irregularities occur in the thickness of the welding seam. Sheets produced in this way were unsuitable for direct further processing into shaped articles by deep-drawing or pressing, because such sheets damage the tool or the shaping operation cannot be carried out with the necessary accuracy.
2. Discussion of Prior Art
In one of the known processes for the butt-welding of galvanized sheets, the welding seam is made by means of an electron beam, with filling material (welding wire) being added. To obtain a quality suitable for further processing, it was necessary to level off the welding seam by means of grinding. Furthermore, in electron-beam welding, a zone without corrosion protection is obtained not only in the region of the welding seam, but also in relatively wide regions next to the welding seam, because the zinc evaporates owing to the high welding temperature. It is therefore necessary, after welding, to provide not only the welding seam, but also the regions stripped of zinc with a corrosion-proof coating. Measures of this type are merely expedients and do not satisfy the fabricators who require a sheet-metal plate protected against corrosion over its entire surface and suitable for deep-drawing.
In another known process for the butt-welding of galvanized sheets and strips by means of a laser beam, the essentially rectangular cut edges are purposely brought to a specific distance from one another, thus producing a continuous welding gap which is filled with molten additive material (welding wire) (EP-A Nos. 0,098,306 and 0,117,751). Such a process, as compared with a butt-welding process by means of an electron beam, is characterized in that the laser beam can be directed more accurately. In this process, therefore, only the welding wire, which can have a larger or smaller diameter than the welding joint, is subjected directly to the laser beam. Consequently, the sheets to be welded to one another are melted only by means of the welding heat of the molten welding wire. The disadvantage of this process is that it produces a relatively wide raised welding seam without corrosion protection, which has to be made flush with the sheet by means of mechanical surface working and which consequently has to undergo protection treatment.